No true Scotsman:

When a justified categorization is rejected based on unconventional or arbitrary criteria.

How can you call yourself a patriot when you are against this war? No true patriot would disagree that this is a just war.

Notes

Those who employ this fallacy will often interject ad hoc or arbitrary criteria to maintain an artificial delineation.

Case Study One

If a theist informs an atheist that his position is indefensible since Stalin was an atheist, it would be a no true Scotsman fallacy for the atheist to suggest that Stalin was not a real atheist. And it would be forfeiting the chance to point out the theist’s own fallacy of top-down condemnation.

Case Study Two

If an atheist informs a Christian that his position is indefensible since Hitler was a Christian, it would be a no true Scotsman fallacy for the theist to suggest that Hitler was not a real Christian. And it would be forfeiting the chance to point out the atheist’s own fallacy of top-down condemnation.